This invention relates to a hollow inert anode having top internal grooves to aid in mechanical attachment to an internal current collector, for use in metal electrolysis processes.
A number of metals including aluminum, lead, magnesium, zinc, zirconium, titanium, and silicon can be produced by electrolysis processes. Each of these electrolytic processes preferably employs an electrode having a hollow interior.
One example of an electrolysis process for metal production is the well-known Hall-Heroult process producing aluminum in which alumina dissolved in a molten fluoride bath is electrolyzed at temperatures of about 960xc2x0 C.-1000xc2x0 C. As generally practiced today, the process relies upon carbon as an anode to reduce aluminum to molten aluminum. Despite the common usage of carbon as an electrode material in practicing the process, there are a number of serious disadvantages to its use, and so, attempts are being made to replace them with inert anode electrodes made of for example a ceramic or metal-ceramic xe2x80x9ccermetxe2x80x9d material.
Ceramic and cermet electrodes are inert non-consumable and dimensionally stable under cell operating conditions. Replacement of carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing costs. Significant environmental benefits are achievable because inert electrodes produce essentially no CO2 or fluorocarbon or hydrocarbon emissions. Some examples of inert anode compositions are found in U.S. Pat. Nos. 4,374,761; 5,279,715; and 6,126,799; 6,217,739; 6,372,119; 6,416,649; 6,423,204 and 6,423,195, all assigned to Alcoa Inc.
Although ceramic and cermet electrodes are capable of producing aluminum having an acceptably low impurity content, they are relatively expensive. Also, to save costs most have a hollow interior into which a conductor rod is sintered/sealed in place. These inert anodes are molded, extruded, or preferably isostatically pressed usually at about 30,000 psi around a smooth round mandrel, which after release of pressure and mandrel removed, provides an unsintered, hollow green anode. This anode must be subsequently fired to sinter it.
In the development of non-metallic, non-consumable electrodes for the production of aluminum and other metals, it is necessary to provide a means of attachment between the conductor, usually metallic, and the non-metallic electrode. This poses technical challenges due to the inherent mis-match in mechanical properties, such as coefficient of thermal expansion, strength and ductility between the two materials. Various solutions have been proposed, including interference fits, locking taper fits, twist and lock arrangements, embedded bolts, and diffusion welding. All of these solutions have one or more severe shortcomings, such as being extremely labor intensive requiring precision machining, relying on precision fits, which exert considerable stress on the brittle electrode material, or requiring long processing time or additional furnace heats.
One example of the inert anode useful in the production of aluminum is shown in FIG. 3 of U.S. Patent Application Publication 2001/0037946 A1 (D""Astolfo Jr. et al.). These anodes operate in a very hot and corrosive environment and must be heated before insertion into a molten cryolite bath.
In one way to make inert anodes, a solid cylindrical mandrel and accompanying flexible mold were used to consolidate ceramic/cermet material into a hollow anode shape through isostatic pressing. After pressing, the mandrel was removed from the anode shape and the shape removed from the mold.
The unfired green part anode shape was then placed upside down (hollow side down) on a firing tray for sintering. After sintering in a kiln, the assembly of an anode was completed.
What is needed is an improved inert anode design that will eliminate the need for inert anode/metal conductor precision fits and relieve stress on the inert anode electrode material. It is a main object of this invention to provide such inert anodes.
The above needs are met and object attained by providing an inert electrode, the electrode having a hollow interior with a top open portion, an interior closed bottom, and side walls, where the interior sidewalls of the top portion have at least one interior groove. The invention also resides in an electrode assembly comprising: (1) an inert electrode having a hollow interior with a top open portion, an interior closed bottom, and side walls, where the interior side walls of the top portion have at least one interior groove; (2) a metal pin conductor having bottom and side surfaces, disposed within the electrode interior but not contacting the electrode interior walls creating an annular gap; and (3) a seal material surrounding the metal pin conductor at the top portion of the electrode, where the seal material fills substantially all of the top annular volume between the at least one interior groove and the top of the conductor, and where a conductive filler material fills at least part of the bottom annular gap between the electrode bottom and the conductor bottom. Preferably, a compliant expansion material is disposed between the conductor and the seal material to protect the seal material from differential thermal expansion. The inert anode material can comprise ceramic, cermet or a metal containing material, such as, for example those described in the above Alcoa patents.
This invention accomplishes a mechanical attachment that is completely internal to the electrode. A support platform can be provided around the conductor pin below seal material, which serves as the primary means of support. Inside the top of the electrode, the circular or other type groove(s) provide, a locking mechanism. The seal material can be a castable ceramic or refractory material to lock the electrode in position relative to the conductor. Also, insulating materials may be added between the castable and conductor or support ring. The advantages of this invention include: no precision machining is required, no precision tolerances are required, there is little or no stress on the electrode material, no additional furnace heats or long process steps are required, and the materials used are inexpensive.